Imaging devices including solid-state image sensors are increasingly being incorporated into handheld devices, such as cameras, mobile cellular telephones and personal digital assistants. A primary design objective is to minimize or reduce the space utilized by the imaging device. Another is to minimize or reduce the cost of the imaging device, and the cost of manufacturing and testing of the imaging device. One reason for these design objectives is that the camera unit provided by the imaging device is often secondary to the main function of the handheld device, such as a mobile telephone.
The imaging device includes an image sensor which comprises a sensor chip and its packaging. The sensor chip has a sensor array formed on a top surface of a die. For protective packaging, the sensor chip is sandwiched between two transparent sheets, typically formed from glass.
The image sensor requires a focussing lens for reproducing an image of the subject. The lens may be a single lens element or a compound lens formed by two or more elements. The lens may be formed from glass or a transparent plastic material or any other material with adequate transparency. The distance from the rear surface of the lens to the image sensor is termed the back focal length.
In conventional imaging devices, the lens is mounted within a support structure commonly referred to as a barrel. The barrel is coupled to a mount that is fixably attached to the top surface of the image sensor, which is the top surface of the upper transparent sheet of the packaging. The lens, barrel and mount are usually made from low cost materials such as plastic.
For variable focus systems (such as autofocus), the barrel, and thus the lens, are movable relative to the mount, and thus the image sensor. The mount is fixed relative to the image sensor. Autofocus systems are more expensive since they require additional components and additional manufacturing and testing steps. They are also relatively large and heavy, and consume relatively large amounts of power.
In many applications, it is acceptable to use a no focus system. In such a case, the barrel and thus the lens, is typically fixed relative to the mount (during initial assembly and testing). The mount is still fixed relative to the image sensor.
The accuracy (allowable tolerance) of positioning of the lens relative to the sensor array of the image sensor depends on the application, but typically is in the range of ±2-100 μm for an acceptable image quality. However, variation in the distance between the lens and the sensor array can arise from a number of sources. There can be variation in the positioning of the lens in the barrel, of the barrel to the mount, of the mount to the image sensor (the thickness of the adhesive used), and of the sensor array within its packaging. There can be manufacturing variations in the lens shape, the height of the mount, and the thickness of the surface of the upper sheet of the packaging.
In known no focus systems, the barrel is typically threadably fastened to the mount. During an assembly and testing stage, the barrel can be rotated a little within the mount to move the barrel closer to or further from the image sensor. This is continued until the image being obtained is deemed to be optimal. The distance between the lens and the image sensor is therefore infinitely variable. However, this approach has drawbacks. First, rotating the barrel within the mount can dislodge foreign material within the threads, and this material can fall onto the sensor array to degrade image quality. Secondly, the infinite variability and subjective assessment of the optimum position significantly increases the testing time for an individual imaging device.